1. Field of the Invention
The invention relates to a fixing temperature control method and an image forming apparatus.
2. Related Background Art
Hitherto, in image forming apparatuses such as printer of an electrophotographic system, copying apparatus, and the like, an image is formed onto a print medium such as paper or the like by electrophotographic processes which are executed in order of a charging process for accumulating charges on a photosensitive drum, an exposing process for irradiating light to a position where image data exists, a developing process for depositing a toner image onto the exposed portion, a transfer process for transferring developed toner onto the print medium, and a fixing process for fixing the toner image on the print medium by heat and pressure (for example, refer to JP-A-10-171192).
FIG. 2 is a diagram showing a construction of the conventional image forming apparatus.
In the diagram, reference numeral 11 denotes a photosensitive drum as an image holding material; 12 a charging roller as a charging apparatus which is arranged around the photosensitive drum 11 and supplies charges onto a surface of the photosensitive drum 11; 13 an exposing apparatus which selectively irradiates light onto the charged surface of the photosensitive drum 11 and forms an electrostatic latent image; 14 a developing roller which deposits toner onto the surface of the photosensitive drum 11 on which the electrostatic latent image has been formed and develops the deposited toner; 15 a toner supplying roller which supplies the toner to the developing roller 14; 16 a transfer belt which conveys the print medium such as paper or the like; 17 a transfer roller which is arranged so as to face the photosensitive drum 11 and transfers the toner image onto the print medium conveyed by the transfer belt 16; and 18 a cleaning blade as a cleaning apparatus which removes the toner remaining on the surface of the photosensitive drum 11.
In the image forming apparatus with such a construction, the charging process for accumulating the charges on the photosensitive drum 11 is executed by the charging roller 12, the exposing process for irradiating the light to the image data forming position on the photosensitive drum 11 is executed by the exposing apparatus 13, the developing process for depositing the toner onto the exposed portion is executed by the developing roller 14, and the transfer process for transferring the developed toner onto the print medium is executed by the transfer roller 17. The fixing process for fixing the toner image on the print medium by the heat and pressure is executed by a fixing device (not shown).
In a color image forming apparatus such as a color electrophotographic printer of a tandem type or the like, drum units of four colors (black, yellow, magenta, and cyan) are arranged and an image of each color is overlaid one by one, thereby forming a final image.
FIG. 3 is a diagram showing a construction of the conventional color image forming apparatus.
In FIG. 3, reference numeral 11BK denotes a photosensitive drum in a drum unit of black; 11Y a photosensitive drum in a drum unit of yellow; 11M a photosensitive drum in a drum unit of magenta; 11C a photosensitive drum in a drum unit of cyan; 17BK a transfer roller in the drum unit of black; 17Y a transfer roller in the drum unit of yellow; 17M a transfer roller in the drum unit of magenta; and 17C a transfer roller in the drum unit of cyan. Each drum unit is constructed as shown in FIG. 2.
Reference numeral 21 denotes an adsorbing portion; 22 a driving roller; 23 an idle roller; 24 a fixing device; and 25 a resist roller.
In the image forming apparatus with such a construction, prior to entering a printing process, a thickness of print medium is measured and process values suitable for the thickness of print medium are determined. In this case, for example, the image forming apparatus has a table of a transfer voltage, a developing voltage, a fixing temperature, and the like which have previously been formed in accordance with the thickness of print medium. The image forming apparatus measures the thickness of print medium and determines the process values such as transfer voltage, developing voltage, fixing temperature, and the like with reference to the table. The thickness of print medium is measured each time the print medium is fed or the thickness of only the first one of the print media stacked on a paper feed tray is measured.
However, in the above conventional image forming apparatus, a throughput of the image forming apparatus deteriorates because of necessity of a waiting time and an inconvenience such as defective image, defective fixing, wrapping, or the like occurs.
That is, the thickness of each print medium is slightly different even in the same kind of print media. Therefore, in the case where the thickness is measured each time the print medium is fed and the process values suitable for the thickness of print medium are decided, it is necessary to take control for increasing or decreasing the transfer voltage, developing voltage, fixing temperature, and the like every print medium. In this case, since it takes time to change the fixing temperature, the waiting time is necessary and the throughput of the image forming apparatus deteriorates.
Therefore, the method of measuring the thickness of only the first one of the print media stacked on the paper feed tray has also been proposed. Since the print media of the same thickness are ordinarily stacked on the paper feed tray, the thickness of the first print medium is almost equal to that of the residual print media. Therefore, there is no need to change the process values such as a fixing temperature and the like. Consequently, there is no need to change the fixing temperature and the like every print medium, the waiting time is unnecessary, and the throughput of the image forming apparatus does not deteriorate.
However, there is a case of presuming a thickness that is largely different from that of the actual print medium due to output errors of sensors, a variation in pages of the print media, or the like. If the process values such as a fixing temperature and the like are determined on the basis of the wrong thickness, the inconvenience such as defective image, defective fixing, wrapping, or the like occurs.
For example, when printing of a high density is performed to a print medium such as thin paper, if a proper fixing temperature is not set, the print medium is likely to be wrapped around the fixing device and the roller will be damaged or broken at worst.